The present invention generally relates to a method and apparatus for controlling electromagnetic emissions. More particularly, the present invention relates to a method and apparatus for controlling electromagnetic emissions through interfaces in electrical assemblies.
Active electrical components emit electromagnetic radiation. The Federal Communications Commission (FCC) regulates the amount of electromagnetic radiation that can be emitted from various classes of electrical and electronic devices by promulgating regulations that dictate the maximum amount of electromagnetic radiation that may be emitted from different types of devices. With respect to computer systems and data storage systems, manufacturers typically control electromagnetic radiation by using conductive enclosures and gaskets to prevent emission of electromagnetic radiation from the various assemblies. Conductive gaskets and conductive enclosures reflect radio frequencies impinging on them and thereby prevent the escape of electromagnetic radiation from the assemblies into the surrounding environment.
Conductive gaskets and enclosures generally are effective at controlling emissions as long as contact between conductive parts of the enclosure is continuous and the interfaces between parts of the enclosure are of low impedance. If contact between parts is not continuous, or if the interfaces between conductive parts are of high impedance, the effectiveness of the conductive enclosure or gasket as an electromagnetic shield will be diminished. Consequently, the effectiveness of the electromagnetic shield may be adversely affected by surface conditions on the enclosure and/or by inadequate contact pressure between the various parts of the assembly (i.e., the enclosure, the gasket material, and any inserted modules within the enclosure).
One problem associated with the use of conductive gaskets is that it is difficult to ensure that adequate contact pressure is maintained between the gasket and the enclosure due to manufacturing variations. In addition to manufacturing variations, mechanical vibrations often make it difficult to ensure that the required constant contact pressure is maintained. Moreover, as materials age, wear, or are damaged, they may lose their effectiveness in keeping the gasket and enclosure in close contact. This aging effect is exemplified by springs made from polymers used in an attempt to force the various conductive parts into contact. As the springs age, the gasket and the enclosure may lose adequate contact pressure. These and other problems make it difficult to ensure that adequate contact will be maintained over time. Therefore, it is difficult to ensure the effectiveness of the conductive shielding over time.
Difficulties with ensuring adequate contact between conductive parts of enclosures surrounding electrical circuit components often significantly increase the overall size, weight, and cost of the assembly since additional structures (e.g., flanges, collars, surface treatment, etc.) are added to ensure contact between the enclosures. Furthermore, the desire to maintain adequate contact between the various electromagnetically shielding structures and/or surface treatments generally affects the appearance of the system assembly and/or subassemblies. For example, in some designs, conductive surfaces are left exposed and are not painted. Moreover, the integration of subassemblies provided by different vendors (e.g., power supplies provided by different vendors) is limited by the requirement that the surface conditions of the various subassemblies provide adequate contact when integrated in the final assembly. Consequently, variations in surface characteristics make it difficult to achieve effective shielding in final assemblies consisting of multiple subassemblies.
Another problem associated with conductive shielding is that, with high frequencies, it is extremely difficult to ensure that contact between subassemblies and the assembly enclosure is adequate to control electromagnetic emissions as well as the ingress of externally generated electromagnetic energy. As frequencies increase, conductive shields become less effective, as the electromagnetic radiation that traverses apertures increases strongly with frequency. Consequently, other shielding methods are used to control high-frequency electromagnetic radiation emissions through various apertures in electrical assemblies.
It is generally known that lossy materials can be used to control electromagnetic radiation. Lossy materials are materials, which absorb, attenuate, and only partially reflect, electromagnetic energy. The ratio of absorption-to-reflection is determined by various properties of the material and by the electromagnetic properties of the energy impinging on the lossy material. However, it is not known to use lossy materials on the outside of enclosures, or housings, and between subassembly housings and a chassis on which the subassembly housings are mounted, to provide an electromagnetic compatibility solution. With respect to these types of system assemblies, conductive gaskets and materials have been used as an EMC solution for a variety of reasons.
Relatively low frequencies can be effectively shielded using electrically conductive gaskets and enclosures made from electrically conductive materials because conductive contact between parts does not have to be continuous in order to ensure effective shielding of emitters operative at these low frequencies. In other words, gaps can exist between parts forming the various electromagnetic shields while still maintaining effective shielding of electromagnetic emitters from subassembly to subassembly and from the assembly to the surrounding environment. Conductive shielding is also effective at preventing the ingress of externally generated low-frequency radiation.
Conductive shielding has been used not only for electromagnetic compatibility, commonly referred to as EMC, but also to ground various subassembly housings in order to eliminate residual voltages which may exist between different parts of the chassis of the system assembly. Thus, conductive shielding has been viewed as advantageous as it performs both of these functions. A system assembly containing electrical circuits that produce a highest expected frequency of 100 MHz, a distance between contact points of 10 cm is often sufficient to pass FCC class A regulations.
Currently, computer system assemblies and data storage system assemblies implement EMC solutions, which use conductive enclosures and conductive gaskets (i.e., conductive shielding). As computer systems and data storage systems are developed which operate at ever increasing frequencies, it can be expected that the problem of undesired electromagnetic emissions will increase over time. Therefore, there is a need for improved systems and methods that address these and/or other shortcomings of the prior art.
EMC solutions suitable for shielding electromagnetic emissions between apertures of an assembly enclosure and the external environment are disclosed. EMC solutions well suited for shielding electromagnetic emissions between the various subassemblies within an assembly enclosure are also presented. In this regard, the EMC solutions include introducing a dielectrically-lossy material in apertures of an assembly enclosure and/or in a gap formed by the external surfaces of subassembly modules and the internal surfaces of an assembly enclosure. The lossy element is made from a material that both absorbs and reflects high-frequency electromagnetic emissions.
Embodiments for shielding various internal subassemblies from electromagnetic emitters in other subassemblies, as well as for preventing electromagnetic emissions from traversing an assembly enclosure include a low-impedance contact that electrically couples a module surrounding each subassembly to an assembly enclosure. The modules and the assembly enclosure are at least partially manufactured of metal, conductive plastic, or plastic with a thin metallization layer. The low-impedance contact is arranged such that the contact substantially overlaps these conductive areas of the subassembly modules and the enclosure.
So configured, an assembly controls both low-frequency and high-frequency energy from entering or exiting gaps between subassemblies and/or between subassembly enclosures and a system chassis with two shielding modes.
Other embodiments of the present invention may be interpreted as providing methods for controlling electromagnetic interference. For example, a preferred method includes the steps of: (1) providing a low-impedance contact to electrically couple an assembly enclosure surrounding a module containing electromagnetic emitting devices to an assembly enclosure or chassis; and (2) introducing at least one lossy element in a channel formed by a gap formed by the inner surfaces of the assembly enclosure and the outer surfaces of the module.
Other systems, methods, and features, of the EMC solution will be apparent to one skilled in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, and features are included within this description, are within the scope of the EMC solution, and protected by the accompanying claims.